Łukasiewicz Łukasz G, Ryu Hye Gun, Mikhaylov Alexander, Azarias Cloé, Banasiewicz Marzena, Kozankiewicz Bolesław, Ahn Kyo Han, Jacquemin Denis, Rebane Aleksander, Gryko Daniel T
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44-52, 01-224, Warsaw, Poland.
Department of Chemistry, POSTECH, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyungbuk, 37673, Korea.
Chem Asian J. 2017 Jul 18;12(14):1736-1748. doi: 10.1002/asia.201700159. Epub 2017 Jun 8.
Five centrosymmetric and one dipolar pyrrolo[3,2-b]pyrroles, possessing either two or one strongly electron-withdrawing nitro group have been synthesized in a straightforward manner from simple building blocks. For the symmetric compounds, the nitroaryl groups induced spontaneous breaking of inversion symmetry in the excited state, thereby leading to large solvatofluorochromism. To study the origin of this effect, the series employed peripheral structural motifs that control the degree of conjugation via altering of dihedral angle between the 4-nitrophenyl moiety and the electron-rich core. We observed that for compounds with a larger dihedral angle, the fluorescence quantum yield decreased quickly when exposed to even moderately polar solvents. Reducing the dihedral angle (i.e., placing the nitrobenzene moiety in the same plane as the rest of the molecule) moderated the dependence on solvent polarity so that the dye exhibited significant emission, even in THF. To investigate at what stage the symmetry breaking occurs, we measured two-photon absorption (2PA) spectra and 2PA cross-sections (σ ) for all six compounds. The 2PA transition profile of the dipolar pyrrolo[3,2-b]pyrrole, followed the corresponding one-photon absorption (1PA) spectrum, which provided an estimate of the change of the permanent electric dipole upon transition, ≈18 D. The nominally symmetric compounds displayed an allowed 2PA transition in the wavelength range of 700-900 nm. The expansion via a triple bond resulted in the largest peak value, σ =770 GM, whereas altering the dihedral angle had no effect other than reducing the peak value two- or even three-fold. In the S →S transition region, the symmetric structures also showed a partial overlap between 2PA and 1PA transitions in the long-wavelength wing of the band, from which a tentative, relatively small dipole moment change, 2-7 D, was deduced, thus suggesting that some small symmetry breaking may be possible in the ground state, even before major symmetry breaking occurs in the excited state.
已通过简单的构建模块以直接的方式合成了五个中心对称和一个偶极的吡咯并[3,2 - b]吡咯，它们含有两个或一个强吸电子硝基。对于对称化合物，硝基芳基在激发态诱导了反转对称性的自发破缺，从而导致大的溶剂化荧光变色现象。为了研究这种效应的起源，该系列采用了外围结构 motif，通过改变 4 - 硝基苯基部分与富电子核心之间的二面角来控制共轭程度。我们观察到，对于二面角较大的化合物，即使暴露于中等极性的溶剂中，荧光量子产率也会迅速降低。减小二面角（即将硝基苯部分与分子的其余部分置于同一平面）可减轻对溶剂极性的依赖性，从而使染料即使在四氢呋喃中也表现出显著的发射。为了研究对称性破缺发生在哪个阶段，我们测量了所有六种化合物的双光子吸收（2PA）光谱和 2PA 截面（σ）。偶极吡咯并[3,2 - b]吡咯的 2PA 跃迁轮廓与相应的单光子吸收（1PA）光谱一致，这提供了跃迁时永久电偶极矩变化的估计值，约为 18 D。名义上对称的化合物在 700 - 900 nm 的波长范围内显示出允许的 2PA 跃迁。通过三键扩展导致了最大的峰值，σ = 770 GM，而改变二面角除了使峰值降低两倍甚至三倍外没有其他影响。在 S → S 跃迁区域，对称结构在带的长波长翼中 2PA 和 1PA 跃迁之间也显示出部分重叠，由此推断出一个试探性的、相对较小的偶极矩变化，为 2 - 7 D，这表明即使在激发态发生主要对称性破缺之前，基态中也可能存在一些小的对称性破缺。
